In recent years, production of semiconductor devices such as blue LEDs, white LEDs, and violet semiconductor lasers by using group 3B nitrides such as gallium nitride and application of such semiconductor devices to various electronic apparatuses have been actively studied. Existing gallium nitride semiconductor devices are mainly produced by vapor-phase methods: specifically, by heteroepitaxial growth of a gallium nitride thin film on a sapphire substrate or a silicon carbide substrate by a metal-organic vapor phase epitaxy method (MOVPE) or the like. In this case, since such a substrate and the gallium nitride thin film are considerably different from each other in terms of thermal expansion coefficient and lattice constant, dislocations (one type of lattice defects in crystals) are generated at a high density in the gallium nitride. Accordingly, it is difficult to provide gallium nitride of high quality having a low dislocation density by vapor-phase methods. Other than vapor-phase methods, liquid-phase methods have also been developed. As an example of liquid-phase methods, a high-temperature high-pressure synthesis method is known. In this method, nitrogen gas is made to be dissolved in molten gallium metal at a high temperature and at a high pressure to thereby directly crystallize gallium nitride. This method provides crystals of high quality; however, the reaction requires a very high temperature such as 1500° C. and a very high pressure such as 1 GPa and hence the method has a problem in practicality. A flux method is one of liquid-phase methods and, in the case of gallium nitride, allows a decrease in the temperature required for gallium nitride crystal growth to about 800° C. and a decrease in the pressure required for gallium nitride crystal growth to several megapascals by using sodium metal as a flux. Specifically, nitrogen gas dissolves in a melt mixture of sodium metal and gallium metal and the melt mixture is supersaturated with gallium nitride and a crystal of gallium nitride grows. Compared with vapor-phase methods, dislocations are less likely to be generated in such a liquid-phase method and hence gallium nitride of high quality having a low dislocation density can be obtained.
Studies on such flux methods have also been actively performed. For example, JP 2007-137735 A proposes a crystal growth method for providing a gallium nitride crystal in which threading defects (defects constituted by dislocations extending through the crystal in the growth direction) are reduced. Specifically, in flux methods, gallium nitride crystals are grown on seed-crystal substrates. Seed crystals are generally generated by vapor-phase methods and hence have a high dislocation density. Accordingly, in flux methods, gallium nitride affected by defects constituted by such dislocations is grown. Such gallium nitride crystals have threading defects extending therethrough in the growth direction. When such gallium nitride crystals are applied to semiconductor devices, threading defects cause a leakage current, which is not preferable. Thus, in JP 2007-137735 A, a gallium nitride crystal is grown on a seed-crystal substrate under conditions such that oblique facets are formed; and the gallium nitride crystal is subsequently grown under conditions such that the crystal has a flat surface and grows in the c-axis direction. As for the latter conditions, the concentration of Li in the melt mixture is controlled to be in the range of 0.5 to 0.8 mol %. As a result, a gallium nitride crystal having a small number of threading defects is obtained.